Upon bacterial infection, host cells release chemicals that damage the bacterial DNA. Such damage, if left unrepaired, leads to bacterial death. Recombination-based DNA repair proteins, specifically the RecBCD and AddAB helicase-nucleases, are excellent targets for a new class of anti- bacterial agents because these proteins 1) are required for the major pathway of DNA repair and are widely distributed in bacteria but apparently absent from eukaryotes, 2) are specifically required during infection by several diverse pathogens, and 3) contribute to induced mutation that causes resistance to existing antibiotics. Drugs against these enzymes should self-limit evolution of bacterial resistance. We will first screen for small-molecule inhibitors of RecBCD in Escherichia coli cells, using a simple, quick, and sensitive assay of nuclease activity in an easily grown E. coli strain. Interesting small molecules will be further tested for inhibition of RecBCD-mediated DNA repair and homologous recombination in E. coli and inhibition of purified enzyme in nuclease and helicase assays. We will use this same approach to look for inhibitors of AddAB from the important gastric pathogen Helicobacter pylori. We have shown that addAB deletion mutants have impaired colonization ability;thus, inhibiting AddAB activity should limit bacterial infections. This approach should allow us to identify drugs against similar enzymes from other pathogenic bacteria. These drugs should limit evolution of bacterial resistance and allow more effective treatment of infectious diseases. PUBLIC HEALTH RELEVANCE: Our long-term objective is to discover and develop a novel class of antibacterial drugs that will provide a new means to combat bacterial infections in the face of bacterial resistance to many currently used antibiotics. Upon bacterial infection, host cells release chemicals that damage the bacterial DNA. Such damage, if left unrepaired, leads to bacterial death. Recombination-based DNA repair proteins are excellent targets for a new class of anti-bacterial agents because inhibitors of these proteins will cripple DNA repair and lead to bacterial death. Drugs against these enzymes should also limit evolution of bacterial resistance and allow more effective treatment of infectious diseases.